This should answer some, but not all, of your questions

Edit: It might actually answer all your questions.

Edit 2: I'll just copy and paste the answers. From /u/banal_penetration

Though it might sound suprising to modern ears, tipping has its origins in Europe, in particular Britain.

In the early modern period it became common for aristocrats to pay a 'vail' when staying as a guest with other aristocrats. This was used to reward servants of that house who put in extra effort and carried out extra duties on behalf of the guests. This spread from there into commercial establishments when coffee shops and restaurants began to expand in the cities. By the 1760s, footmen, valets and other staff would expect vails, and there are references in a number of contemporary sources to tipping in coffee houses.

Prior to the Civil War, Americans did not tip. Much like in modern Japan, it was seen as unseemly to do so, undemocratically highlighting the social power between tipper and tippee. This changed in the late 19th century when Americans started visiting Europe. They picked up the practice and quickly became renowned for over-tipping - partly seen as a consequence of them not being used to having staff in the way that wealthy Europeans were.

When Americans returned home, the practice spread, though it was not without critics. Anti-Tipping leagues were formed, and the practice was banned in 6 states between 1909 and 1926.

In the 1960s tipping became even more common and arguable mandatory when Congress altered the law to allow minimum wage to be discounted for tips. This forced many waitstaff to live off tips, and therefore they became a greater expectation. The growth of people having a waiting job at some point in their lives also encouraged the growth of tipping culture.

If you want to know more, the best book on the subject is probably Tipping: An American Social History of Gratuities by Kerry Segrave though it is more of an archive than analysis.

From /u/scoopny:

Waitstaff are not the only workers who were subject to a lower minimum wage, farm workers and domestic workers were also subject to a lower minimum wage. The reasons? Southern politicians would not support a minimum wage that did not exempt job classes that employed large numbers of African Americans. In short, they did not want to be required to pay black workers the same wages as white workers. Source: Fear Itself the New Deal and the Origins of Our Time.